Adds support for distros that use busybox like Alpine

Checks for busybox by looking at the realpath for timeout
If busybox is detected, uses timeout with a -t option and
uses nc instead of /bin/tcp

Author: iturgeon

wait-for-it.sh

@@ -32,8 +32,13 @@ wait_for()
     start_ts=$(date +%s)
     while :
     do
-        (echo > /dev/tcp/$HOST/$PORT) >/dev/null 2>&1
-        result=$?
+        if [[ $ISBUSY -eq 1 ]]; then
+            nc -z $HOST $PORT
+            result=$?
+        else
+            (echo > /dev/tcp/$HOST/$PORT) >/dev/null 2>&1
+            result=$?
+        fi
         if [[ $result -eq 0 ]]; then
             end_ts=$(date +%s)
             echoerr "$cmdname: $HOST:$PORT is available after $((end_ts - start_ts)) seconds"
@@ -48,9 +53,9 @@ wait_for_wrapper()
 {
     # In order to support SIGINT during timeout: http://unix.stackexchange.com/a/57692
     if [[ $QUIET -eq 1 ]]; then
-        timeout $TIMEOUT $0 --quiet --child --host=$HOST --port=$PORT --timeout=$TIMEOUT &
+        timeout $BUSYTIMEFLAG $TIMEOUT $0 --quiet --child --host=$HOST --port=$PORT --timeout=$TIMEOUT &
     else
-        timeout $TIMEOUT $0 --child --host=$HOST --port=$PORT --timeout=$TIMEOUT &
+        timeout $BUSYTIMEFLAG $TIMEOUT $0 --child --host=$HOST --port=$PORT --timeout=$TIMEOUT &
     fi
     PID=$!
     trap "kill -INT -$PID" INT
@@ -136,6 +141,17 @@ STRICT=${STRICT:-0}
 CHILD=${CHILD:-0}
 QUIET=${QUIET:-0}
 
+# check to see if timeout is from busybox?
+# check to see if timeout is from busybox?
+TIMEOUT_PATH=$(realpath $(which timeout))
+if [[ $TIMEOUT_PATH =~ "busybox" ]]; then
+        ISBUSY=1
+        BUSYTIMEFLAG="-t"
+else
+        ISBUSY=0
+        BUSYTIMEFLAG=""
+fi
+
 if [[ $CHILD -gt 0 ]]; then
     wait_for
     RESULT=$?